The present invention relates to integrated circuits and methods of manufacturing integrated circuits. More particularly, the present invention relates to a method of manufacturing integrated circuits having transistors with ultra-shallow source/drain extensions.
Integrated circuits (ICs), such as, ultra-large scale integrated (ULSI) circuits, can include as many as one million transistors or more. The ULSI circuit can include complementary metal oxide semiconductor (CMOS) field effect transistors (FETS). The transistors can include semiconductor gates disposed between drain and source regions. The drain and source regions are typically heavily doped with a P-type dopant (boron) or an N-type dopant (phosphorous).
The drain and source regions generally include a thin extension that is disposed partially underneath the gate to enhance the transistor performance. Shallow source and drain extensions help to achieve immunity to short-channel effects which degrade transistor performance for both N-channel and P-channel transistors. Short-channel effects can cause threshold voltage roll-off and drain-inducted barrier lowering. Shallow source and drain extensions and, hence, controlling short-channel effects, are particularly important as transistors become smaller.
Conventional techniques utilize a double implant process to form shallow source and drain extensions. According to the conventional process, the source and drain extensions by providing a transistor gate structure without sidewall spacers on a top surface of a silicon substrate. The silicon substrate is doped on both sides of the gate structure via a conventional doping process, such as, a diffusion process or ion implantation process. Without the sidewall spacers, the doping process introduces dopants into a thin region (i.e., just below the top surface of the substrate) to form the drain and source extensions as well as to partially form the drain and source regions.
After the drain and source extensions are formed, silicon dioxide or silicon nitride spacers, which abut lateral sides of the gate structure, are provided over the source and drain extensions. The substrate is doped a second time to form the deeper source and drain regions. The source and drain extensions are not further doped due to the blocking capability of the silicon dioxide spacer.
As transistors disposed on integrated circuits (ICs) become smaller, transistors with shallow and ultra-shallow source/drain extensions have become more difficult to manufacture. For example, transistors with physical gate lengths smaller than 100 nm should have ultra-shallow source and drain extensions (less than 30 nanometer (nm) junction depth) according to 1997 national semiconductor technology load map. Forming source and drain extensions with junction depths of less than 30 nm is very difficult using conventional fabrication techniques. Conventional ion implantation and diffusion doping techniques make transistors on the IC susceptible to short-channeling effects, which result in a dopant profile tail distribution that extends deep into the substrate. Also, conventional ion implantation techniques have difficulty maintaining shallow source and drain extensions because point defects generated in the bulk semiconductor substrate during ion implantation can cause the dopant to more easily diffuse (transient enhanced diffusion, TED). The diffusion often extends the source and drain extension vertically in to the bulk semiconductor substrate.
Thus, there is a need for a method of manufacturing ultra-shallow source and drain extensions that does not utilize a conventional double implant process. Further still, there is a need for transistors that have ultra-shallow junction source and drain extensions. Even further still, there is a need for an efficient method of manufacturing source and drain extensions.
The present invention relates to a method of manufacturing an integrated circuit that includes depositing a solid phase impurity source and thermally annealing a substrate. The solid phase impurity source is provided over a gate structure disposed on the semiconductor substrate. The substrate is thermally annealed to drive a dopant from the solid phase impurity source into the semiconductor substrate, thereby forming source and drain extensions.
The present invention still further relates to a method of manufacturing an ultra-large scale integrated (ULSI) circuit. The integrated circuit includes a plurality of field effect transistors having shallow source and drain extensions. The method includes the steps of forming at least part of a gate structure on a top surface of a semiconductor substrate, depositing a doped silicon dioxide over the top surface of the semiconductor substrate, and thermally annealing the substrate. Thermally annealing the substrate drives a dopant from the doped silicon dioxide into the semiconductor substrate, thereby forming source and drain extensions having a thickness or junction depth less than 30 nanometers (nm).
The present invention further still relates to a method of providing a plurality of ultra-shallow drain/source extensions for field effect transistors associated with an ultra-large scale integrated circuit. The method includes forming at least a portion of a gate structure on a top surface of a silicon substrate, depositing a solid phase impurity source over the top surface, and driving dopants from the impurity source into the substrate to form the ultra-shallow drain/source extensions.